There's a lot that kids can learn, from a social point of view, in your 'average' comprehensive, the value of which I don't think should be overlooked. Two years I spent at my local high school, from 11 to 13 were some of the best of my education from the point of view of interacting with a wide range of different personalities, but also in learning the real value of education, and my own responsibility in ensuring I got the most out of it.
The teaching at the selective grammar I moved on to at 13 was great as you'd expect, but I really noticed the narrowing in the demographic of my peers.
The red brick university I went on to gave a great balance between learning and the socialising/mixing with a wide range of people that I think can be just as important, but I found moving on to Cambridge for a further degree to be a huge culture shock in comparison. Many of the undergrads I helped to support/teach, particularly those that came from private schools, had an expectation to be spoon-fed just what they needed, and less motivation to learn independently. I am of course generalising, no two schools, whether state or private are the same, but these are my overall observations. I'm also well aware that I've undoubtedly benefitted from selective schools, albeit academically selective, rather than monetarily.
Above all, you know your own children best, so should be able to judge what would benefit them the most. Some children can thrive in a boisterous school environment, whilst others might do better with a more managed environment. Just be aware that a more selective school has its pluses (better teaching/facilities), but you can lose out in other areas.